Olefin epoxidation process

ABSTRACT

A process for the production of an olefin oxide, which process comprises reacting a feed comprising an olefin and oxygen in a reactor tube in the presence of a silver-containing catalyst, wherein the presence of water in the catalyst bed is controlled such that the ratio of the partial pressure of water (PPH 2 O) divided by the vapor pressure of water (VPH 2 O) is less than 0.006, preferably less than 0.004.

The present application is a continuation of U.S. application Ser. No.13/246,325 filed Sep. 27, 2011, which claims the benefit of U.S.Provisional Application No. 61/387,858 filed Sep. 29, 2010, both ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a process for the production of anolefin oxide, a 1,2-diol, a 1,2-diol ether, 1,2-carbonate or analkanolamine.

BACKGROUND OF THE INVENTION

In olefin epoxidation an olefin is reacted with oxygen to form an olefinepoxide, using a catalyst comprising a silver component, usually withone or more further elements deposited therewith on a support. Theolefin oxide may be reacted with water, an alcohol, carbon dioxide or anamine to form a 1,2-diol, a 1,2-diol ether, 1,2-carbonate or analkanolamine. Thus, 1,2-diols, 1,2-diol ethers, 1,2-carbonates andalkanolamines may be produced in a multi-step process comprising olefinepoxidation and converting the formed olefin oxide with water, analcohol, carbon dioxide or an amine.

The performance of the epoxidation process may be assessed on the basisof the selectivity, the catalyst's activity and stability of operation.The selectivity is the molar fraction of the converted olefin yieldingthe desired olefin oxide. The catalyst is subject to an ageing-relatedperformance decline during normal operation. The ageing manifests itselfby a reduction in the activity of the catalyst. Usually, when areduction in activity of the catalyst is shown, the reaction temperatureis increased in order to compensate for the reduction in activity,however at the expense of selectivity. In the typical operation of afresh catalyst, the process is operated at a reaction temperature of upto about 250° C. Upon catalyst ageing the reaction temperature maygradually be increased to values substantially above 250° C. until thereaction temperature becomes undesirably high or the selectivity becomesundesirably low, at which point in time the catalyst is deemed to be atthe end of its lifetime and would need to be exchanged. It goes withoutsaying that from an economical point of view it is highly desirable toimprove the performance of the catalyst and to extend its lifetime asmuch as possible. Quite modest improvements in the maintenance ofselectivity over long periods yield huge dividends in terms ofefficiency in the olefin epoxidation process and, if applicable, also inthe overall process for the production of a 1,2-diol, a 1,2-diol ether,1,2-carbonate or an alkanolamine.

Therefore, for decades much research has been devoted to improving theactivity, the selectivity and the lifetime of the catalysts, and to findprocess conditions which enable full exploitation of the catalystperformance. For example, it is well known that low CO₂ levels areuseful in improving the selectivity of high selectivity catalysts. See,e.g., U.S. Pat. No. 7,237,677; U.S. Pat. No. 7,193,094; US 2007/0129557;WO 2004/07873; WO 2004/07874; and EP 2,155,708. These patents alsodisclose that water concentration in the reactor feed should bemaintained at a level of at most 0.35 mole percent, preferably less than0.2 mole percent. Other patents disclose control of the chloridemoderator to maintain good activity. See, e.g., U.S. Pat. No. 7,657,331;EP 1,458,698; and US Pub. Pat. App. 2009/0069583. Still further, thereare many other patents dealing with the epoxidation process and means toimprove the performance of the catalyst in the process. See, e.g., U.S.Pat. Nos. 7,485,597, 7,102,022, 6,717,001, 7,348,444, and US Pub. Pat.App. 2009/0234144.

Notwithstanding the improvements already achieved, there is a desire tofurther improve the performance of the silver-containing catalysts inthe production of an olefin oxide, a 1,2-diol, a 1,2-diol ether, a1,2-carbonate or an alkanolamine.

SUMMARY OF THE INVENTION

The present invention provides a process for the production of an olefinoxide, which process comprises reacting a feed comprising an olefin andoxygen in the presence of a supported silver-containing catalyst loadedinto a reactor tube (i.e., the catalyst bed), wherein the presence ofwater at any point in the catalyst bed is controlled such that the ratioof the partial pressure of water (PPH₂O) divided by the vapor pressureof water (VPH₂O) is less than 0.006, preferably less than 0.004. Asshown in the examples which follow, even the low levels of water thathad been considered acceptable in the past are detrimental to theperformance of a silver-containing catalyst.

This invention constitutes a means to reduce the rate of selectivityloss by an epoxidation catalyst while in operation that is differentfrom the well-known effects of time and temperature described above inthe prior art. In this invention the water vapor concentration in thecatalyst bed is reduced to certain levels in order to reducesignificantly the rate of selectivity loss and the overall selectivityloss during the catalyst operational cycle. This is different from theprior art view, because the primary effect of reducing water vaporconcentration is not a slower decline rate due to lower operatingtemperature for the catalyst. In this invention, we have found thatwater causes another ageing mechanism which may actually lower the rateof temperature increase but at the same time cause acceleratedselectivity loss. In this invention we have found that significantlylower concentrations of water in the vapor phase at conditions wherecondensation of liquid water is not possible has resulted in changes inthe catalyst that lead to loss of selectivity. The hygroscopic nature ofthe catalyst or catalyst support results in adsorption of water on thesurface of the catalytic material even when conditions are such thatliquid water should not be present on the catalyst or internal reactorsurfaces. i.e., well above the dew point of water. Thus, the presence ofexcess water in the vapor phase will suppress selectivity and lead toincreased rates of sintering or losses of key water soluble dopants fromthe catalyst surface.

In the present invention, we have found that the redistribution of keywater soluble dopants on the surface of the catalyst can be greatlyreduced and therefore the catalyst selectivity loss rates can be reducedsignificantly by reduction of the ratio of the partial pressure of water(PPH₂O) divided by the vapor pressure of water (VPH₂O) at the inlet andthroughout the catalyst bed. Vapor phase water is introduced in atypical commercial reactor in the feed gas at the inlet of the reactoras well as by generation within the reactor due to the completecombustion of a portion of the ethylene fed to the reactor to CO₂ andwater. See, e.g., US Pub. Pat. App. 2009/0234144, which disclosure isincorporated in its entirety herein. There are a number of ways by whichthe ratio of the partial pressure of water (PPH₂O) divided by the vaporpressure of water (VPH₂O) can be reduced. These include:

-   -   Increased cooling of the overhead streams coming from the        ethylene oxide (“EO”) removal and/or CO₂ removal sections of the        plant that return to the reactor.    -   Diversion of less of the recycle gas through the CO₂ absorber.    -   Operation of the EO and CO₂ absorbers at lower temperature.    -   Increasing the Gas Hourly Space Velocity at fixed EO production        to reduce the water concentration gradient increase in the        reactor.    -   Reduction in work rate or EO production per unit volume of        catalyst to reduce the amount of H₂O formed in the reactor.    -   Utilization of catalysts with higher selectivity such that the        amount of water produced across the catalyst bed is reduced for        a given EO production rate.    -   Reducing the reactor operating pressure so as to reduce the        partial pressure of H₂O.    -   Operation of the reactor at higher temperature than required to        increase the vapor pressure of water.

These are some of the means by which the ratio of the partial pressureof water (PPH2O) divided by the vapor pressure of water (VPH2O) in thereactor/catalyst bed can be reduced, but this is not an exhaustive list.The concept can be applied to existing plants by making changes inoperating variables and/or changes to plant hardware such as heatexchangers, absorbers, and compressors. The concept can be applied tonew plants in the design phase as well.

A quantitative analysis method has been developed to determine the levelof water vapor which causes accelerated selectivity loss of epoxidationcatalysts while in operation. Extensive evaluation of post mortemresults of spent catalysts demonstrated that surface concentrations ofwater soluble dopants as measured by X-ray Photoelectron Spectroscopy(“XPS”) were reduced significantly when the ratio of the water partialpressure in the gas phase to the vapor pressure (PPH₂O/VPH₂O) of waterat the location of the sample in the reactor during operation exceeded0.004. The reduction of surface concentration of these water solubledopants is directly linked to selectivity loss of the catalyst. Sampleswhich were not exposed to PPH₂O/VPH₂O>0.004 showed much less reductionin the surface concentration of water soluble dopants and much lessselectivity loss. It is most preferable that the ratio of PPH₂O/VPH₂O isless than 0.004 over the entire length of the catalyst bed. Butadvantages are also shown where the ratio is less than 0.004 over aportion of the catalyst bed—for example where the ratio is less than0.004 over greater than 50% of the reactor tube length (defined as thelength from the catalyst bed inlet to the catalyst bed outlet),preferably over greater than 80% of the reactor bed length.

While ratios of PPH₂O/VPH₂O<0.004 are highly desirable, it may not bepossible to achieve this in many commercial plants throughout the entirecatalyst bed due to hardware limitations, operating constraints, or EOproduction requirements. This does not preclude a plant from takingadvantage of the concept. Reduction of PPH₂O/VPH₂O is expected to bebeneficial no matter what the starting point. Thus, if a plant canreduce PPH₂O/VPH₂O from 0.007 to 0.006 one would still expect abeneficial effect. Likewise, it may be possible to increase the portionof the catalyst bed that operates at PPH₂O/VPH₂O<0.004. This will have abeneficial effect as it will reduce the rate of selectivity loss fromthis portion of the bed and will have a beneficial effect on the rest ofthe catalyst bed which may operate above this threshold.

The finding that water vapor concentrations can cause acceleratedselectivity loss is an unexpected result as the traditional view hasbeen that higher temperatures were the primary driving force behindselectivity loss. Thus, the invention described here is fundamentallydifferent from prior methods attempted to increase initial selectivityand to reduce the rate of selectivity decline for a given catalyst.Another unexpected aspect of the invention is that the moderator levelneeds to be shifted, usually towards higher levels, as the water levelis decreased, in order to maintain optimum performance. This is notintuitive—water level has little overall impact on catalyst activity,and the “Q” factor optimum is primarily a function of temperature, somost operators wouldn't see any reason to change the “Q” factor withwater levels. Those that do would change the “Q” factor proportionallyto the water level, in order to compensate for H₂O adsorption, but inmost situations, this change is in the wrong direction. The “Q” factoris taught in U.S. Pat. No. 7,193,094, which patent is incorporated byreference herein. However, in the '094 patent, the moderator level isprimarily a function of the reactor temperature. This current inventionreveals that proper control of the “Q” factor requires re-optimizationof the moderator level whenever any significant shift in water levelsoccurs in the catalyst bed.

The invention also provides a method of using an olefin oxide for makinga 1,2-diol, a 1,2-diol ether, 1,2-carbonate or an alkanolaminecomprising converting the olefin oxide into the 1,2-diol, the 1,2-diolether, 1,2-carbonate or the alkanolamine, wherein the olefin oxide hasbeen obtained by the process according to this invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 depicts the relationship between selectivity and relative surfaceconcentration of cesium at various ratios of the partial pressure ofwater divided by the vapor pressure of water for Plant W.

FIG. 2 depicts the relationship between selectivity and relative surfaceconcentration of cesium at various ratios of the partial pressure ofwater divided by the vapor pressure of water for Plant X.

FIG. 3 depicts the relationship between selectivity and relative surfaceconcentration of cesium at various ratios of the partial pressure ofwater divided by the vapor pressure of water for Plant Y.

DETAILED DESCRIPTION OF THE INVENTION

I. Method to Calculate PPH₂O/VPH₂O

The following steps provide a full description of the methodologyrequired to calculate the partial pressure of water in the gas phase atmultiple positions within the catalyst bed of a reactor tube and thevapor pressure of water at each axial position as well. Finally, theratio of water partial pressure to water vapor pressure is calculated sothat it can be determined if the water concentration in the gas phase ateach axial position will cause increased selectivity loss.

Step A. Measure or Estimate the Axial Gas Temperature Profile in theCatalyst Bed.

The axial gas temperature profile within a tube packed with catalyst canbe measured directly with thermocouples placed in selected tubes in thereactor. In many cases, 5-20 tubes in the reactor will have an internalcatalyst bed thermocouple sheath installed along the length of thereactor. Each thermocouple sheath typically has 5-10 temperatureindicating points at known positions along its length. Thesethermocouples placed in a select number of tubes provide a closeapproximation of the gas temperature in the remaining tubes in thereactor.

In case the reactor does not have thermocouples placed in the catalystbed to measure the axial gas temperature profile, the gas temperaturecan be estimated from the outlet gas temperature (“OGT”), and coolanttemperature measurements. For example in one case, the measured OGT was242.9° C. and the following coolant temperature measurements wereavailable even though a thermocouple was not placed in the catalyst bed.

1.33 m 2.33 m 3.57 m 4.57 m 5.73 m 6.73 m 7.07 m 9.07 m 10.33 m 11.33 mCoolant Coolant Coolant Coolant Coolant Coolant Coolant Coolant CoolantCoolant 235.0 235.2 235.4 235.5 235.6 235.7 235.9 236.0 236.3 236.4

Typically at a distance of 1 meter into the catalyst bed, the gastemperature will be equal to the coolant temperature. At subsequentdistances into the reactor, the gas temperature in the catalyst bed willexceed the coolant temperature by ˜1-15° C. in reactors using water as acoolant and by 5-30° C. in reactors using a hydrocarbon coolant. Evenhigher differences between the gas temperature and the coolant can occurin situations where subcooled coolant is supplied to the reactor or thereactor is being operated at severe conditions. To approximate the gastemperature in the catalyst bed of a water cooled reactor, a constantoffset can be added to the coolant temperatures beyond the 1 meterdistance. The difference between the coolant temperature at the outletof the reactor and the OGT provides a close approximation provided theOGT is measured before significant cooling of the reactor product gasoccurs.

Offset=OGT−11.33 m Coolant Temperature

Offset=242.9° C.−236.4° C.=6.5° C.

Thus, the gas temperatures in the catalyst bed would be:

1.33 m Catalyst=235.0° C.+6.5° C.=241.5° C.

2.33 m Catalyst=235.2° C.+6.5° C.=241.7° C.

3.57 m Catalyst=235.4° C.+6.5° C.=241.9° C.

11.33 m Catalyst=236.4° C.+6.5° C.=242.9° C.

In the case of a boiling hydrocarbon cooled reactor, there is often alarger difference between outlet gas and coolant temperatures at theoutlet of the reactor and the difference between coolant temperature andgas temperature in the catalyst bed may increase as the distance fromthe inlet of the catalyst bed increases. As an example, if: OGT=250° C.for a hydrocarbon cooled reactor and coolant temperature near the outletof the reactor was 238° C., then the following maximum offset would befound at the reactor outlet:

Maximum Offset=OGT−temperature at 11.33 meter for coolant=250° C.−238.0°C.=12° C.

In most cases a good approximation to the axial gas temperature profilethrough the catalyst bed can be obtained by assuming a linearlyincreasing catalyst temperature from near the inlet of the catalyst bedto the outlet of the bed. The gas temperature profile can be estimatedby the following equation:

Gas Temperature(z)=Coolant Temperature(z)+(Maximum Offset)times((z−z_(heatup))/(L−z _(heatup)) where:

-   -   Gas Temperature(z) is the temperature of the gas within the        catalyst bed at the axial distance, z as measured from the inlet        of the bed.    -   Coolant Temperature (z) is the coolant temperature at position        z.    -   Maximum Offset is described above.    -   z_(beatup) is the length of the tube required to bring the inlet        gas up to the coolant temperature. Typical is approximately 1        meter.    -   L is the total length of the catalyst bed.

Other methods may be used to measure or estimate the gas temperature inthe catalyst bed depending on the reactor configuration, mode ofoperation, and available measurements of actual conditions.

Step B. Measure or Estimate the Axial Pressure Profile within theCatalyst Bed

a. The pressure of the gas in the catalyst bed must be measured orestimated at each point for which it is desired to calculate the partialpressure of the H₂O vapor in the catalyst bed.

b. Normally, the gas pressure is measured at the inlet and outlet of thereactor by pressure-indicating transducers, gauges or other devices.

c. The gas pressure throughout the catalyst bed can be closelyapproximated by assuming the pressure changes linearly with position inthe catalyst bed from the inlet value to the outlet value.

d. The following equation can be applied to estimate the pressure ateach axial position if the inlet and outlet pressure values are known.

Pressure(z)=Inlet Pressure+(Outlet Pressure−Inlet Pressure)times(z/L)

Where:

-   -   1. Pressure (z) is the pressure at position z within the        catalyst bed.    -   2. Inlet Pressure is the absolute pressure as measured at the        inlet of the reactor.    -   3. Outlet Pressure is the absolute pressure as measured at the        outlet of the reactor.    -   4. Z is the distance from the inlet of the catalyst bed.    -   5. L is the overall length of the catalyst bed.

Other methods or models may be applied to calculate the axial pressureprofile in the reactor if outlet pressure measurements are notavailable.

Step C. Calculate the Axial Water Partial Pressure Profile within theCatalyst Bed.

a. Given the water concentrations in the gas phase entering and exitingthe reactor along with the axial pressure profile determined in Step B,the axial profile of the water partial pressure can be calculated by thefollowing equation.

b. In the following calculation, it is assumed that the mole fraction ofH₂O increases linearly from the inlet to the outlet of the catalyst bed.Experimental data have shown this to be a reasonable assumption.

PPH₂O(z)=([H₂Oin]+([H₂Oout]−[H₂Oin])times(z/L))times Pressure(z) where:

PPH₂O(z) is the partial pressure of water in the gas phase a distance zfrom the inlet of the catalyst bed.

[H₂Oin] is the mole fraction of water in the gas phase at the entranceto the catalyst bed. [H₂Oin] can be measured by a number of analyticalmethods, including the well-known Karl Fischer Titration method (ASTME203-08) or calculated based on process knowledge and measurements oftemperature, pressure, and flow in the process.

[H₂Oout] is the mole fraction of water in the gas leaving the catalystbed and can be measured by a number of analytical techniques, includingASTM E203-08.

In the absence of actual measurements of [H₂Oout], the stoichiometry ofthe complete combustion of ethylene to CO₂ and H₂O can be used tocalculate accurately the outlet water mole fraction provided the amountof CO₂ formed in the reactor has been measured and the inlet H₂O molefraction is known. For each mole of CO₂ formed in the reactor, one moleof H₂O will be formed. In these instances, [H₂Oout] can be calculatedaccurately as follows:

[H₂Oout]=[H₂Oin]+([CO₂out]−[CO₂in]) where:

[CO₂out] is the mole fraction of CO₂ out of the reactor

[CO₂in] is the mole fraction of CO₂ into the reactor

Step D. Calculate the Water Vapor Pressure Axial Profile

a. The vapor pressure of water can be calculated at each axial positionwithin the catalyst bed using the following correlation.

VPH₂O(bara)=exp(A+B/T+C ln(T)+DT̂E)times 10⁻⁵ where

VPH₂O is the vapor pressure of water in bara

A=73.649

B=−7258.2

C=−7.3037

D=0.0000041653

E=2

T is the gas temperature in ° K

-   Reference: Gallagher, J. S., Haar, L., Kell, G. S, NBS/NRC Steam    Tables. Thermodynamic and Transport Properties and Computer Programs    for Vapor and Liquid States of Water in SI Units. Hemisphere Publish    Corporation, Washington, 1984.

The results of an example calculation are shown below.

Axial Position (z) 1.33 2.33 3.57 4.57 5.73 6.73 7.07 9.07 10.33 11.33 mGas Pressure in 19.1 18.9 18.6 18.4 18.2 17.9 17.9 17.4 17.2 16.9 barathe Catalyst Bed Gas Temperature 236.6 238.0 239.7 241.0 242.5 243.8244.3 246.9 248.6 250.0 ° C. in Catalyst Bed Gas Temperature 509.8 511.1512.8 514.1 515.6 516.9 517.5 520.1 521.8 523.2 ° K in Catalyst BedWater Vapor 31.5 32.3 33.2 34.0 34.9 35.7 36.0 37.7 38.8 39.7 baraPressure

Step E. Calculate the Ratio of PPH₂O to VPH₂O

Calculation of the ratio of water partial pressure in the gas phaserelative to the vapor pressure is a straightforward calculation from theresults of Steps C and D.

Ratio=PPH₂O(z)/VPH₂O(z) where:

PPH₂O(z) is the partial pressure of water at a distance z from the inletof the catalyst bed.

VPH₂O(z) is the vapor pressure of water at a distance z from the inletof the catalyst bed.

The results of an example calculation are shown below.

Axial Position (z), m 1.33 2.33 3.57 4.57 5.73 6.73 7.07 9.07 10.3311.33 m Water Partial 0.111 0.121 0.133 0.143 0.153 0.162 0.165 0.1820.192 0.200 bara Pressure Water Vapor 31.5 32.3 33.2 34.0 34.9 35.7 36.037.7 38.8 39.7 bara Pressure Ratio 0.00352 0.00375 0.00401 0.004190.00439 0.00454 0.00458 0.00483 0.00495 0.00503 PPH2O/VPH2O

If the values in the table above exceed 0.004, then it would be expectedthat the catalyst will show accelerated decline resulting from thepresence of water.

II. Process for Making Olefin Oxide

Although the present epoxidation process may be carried out in manyways, it is preferred to carry it out as a gas phase process, i.e. aprocess in which the feed is contacted in the gas phase with thecatalyst which is present as a solid material, typically in a packedbed. Generally the process is carried out as a continuous process.

The olefin for use in the present epoxidation process may be any olefin,such as an aromatic olefin, for example styrene, or a di-olefin, whetherconjugated or not, for example 1,9-decadiene or 1,3-butadiene.Typically, the olefin is a mono olefin, for example 2-butene orisobutene. Preferably, the olefin is a mono-α-olefin, for example1-butene or propylene. The most preferred olefin is ethylene.

The olefin content of the feed is typically between 15 and 50 molepercent, relative to the total feed. In preferred embodiments, amongstothers, the olefin content of the feed is maintained at a value of atleast 25 mole-%. Typically the olefin content of the feed is maintainedat the value as defined for at least a period which is sufficient toeffect an olefin oxide production of at least 1,000 kmole, moretypically at least 5,000 kmole, most typically at least 10,000 kmole, ofolefin oxide per m³ catalyst bed, preferably up to the end of thecatalyst's lifetime, that is when the catalyst will be exchanged and/orrejuvenated. As used herein, the feed is considered to be thecomposition which is contacted with the catalyst.

The direct oxidation of an olefin to the corresponding olefin oxide canbe air-based or oxygen-based, see Kirk-Othmer's Encyclopedia of ChemicalTechnology, 3rd ed., Vol. 9 (1980) p. 445 to 447, and Encyclopedia ofCatalysts, Vol. 3 (2003) p. 246-264. In the air-based processes air orair enriched with oxygen is fed directly to the system while in theoxygen-based processes high-purity (above 95 mol-%) oxygen is employedas the source of the oxidizing agent. Presently most ethylene oxideproduction plants are oxygen-based and this is the preferred embodimentof the present invention.

The oxygen content of the feed is within the broad range of from 3 to 20mole-%, preferably from 5 to 12 mole-%, relative to the total feed.

In order to remain outside the flammability limit of the reactionmixture, the oxygen content of the feed is usually balanced with theolefin content. The actual safe operating ranges depend, along with thegas composition (reactants and balance gases), also on individual plantconditions such as temperature and pressure.

In addition to the olefin and oxygen, the feed may contain one or moreoptional components, such as carbon dioxide, a reaction modifier(moderator), a reaction co-modifier (co-moderator) and balance inertgases.

Carbon dioxide is a by-product of the olefin oxidation process. Sinceunconverted olefin is continuously recycled, and since carbon dioxide inthe feed will have an adverse effect on catalyst activity, accumulationof carbon dioxide will be avoided by continuously removing carbondioxide from the recycle gas. This may be done by venting and bycontinuous absorption of the formed carbon dioxide. Currentlyconcentrations of carbon dioxide in the feed gas stream as low as0.2-0.3 mole-% are practical, though amounts of as high as 3 mole-% areoften found in practice.

Reaction moderators and co-moderators may be added to the feed forincreasing the selectivity, suppressing the undesirable oxidation ofolefin and of the olefin oxide to carbon dioxide and water. Many organiccompounds, especially organic halides but also amines, ammonia,organometallic compounds and aromatic hydrocarbons are known to beeffective in this respect. Organic halides are the preferred reactionmoderators and they are effective without suppressing the desiredreaction when used in quantities ranging from 0.1 to 25 parts permillion by volume (ppmv), in particular from 0.3 to 20 ppmv, relative tothe total feed. Dependent of the silver-containing catalyst employed,the reaction moderator content of the feed may be optimized from time totime during operation if the maximum achievable selectivity is to bemaintained. Preferred organic halides are C₁ to C₈ chloro hydrocarbonsor bromo hydrocarbons. More preferably they are selected from the groupof methyl chloride, ethyl chloride, ethylene dichloride, ethylenedibromide, vinyl chloride or a mixture thereof. Most preferred reactionmoderators are ethyl chloride and ethylene dichloride.

Typically, as the organic halide moderator level is increased, thecatalyst activity decreases, and the catalyst selectivity passes througha maximum. Therefore, typical operation of an ethylene oxide plantinvolves maintaining the moderator level to maintain this optimumselectivity. As disclosed in U.S. Pat. No. 7,193,094, as the hydrocarboncomposition changes, the moderator level has to change as well, in orderto maintain operation at maximum selectivity. Likewise, as the catalysttemperature increases, the moderator level also needs to increase. Thiscurrent invention teaches that as the water concentration in thecatalyst bed changes, the moderator level needs to be re-optimized aswell, regardless of whether or not the catalyst temperature orhydrocarbon composition has changed. It has been found that with freshcatalysts, increasing the water level led to almost no change incatalyst activity (i.e., operating temperature at a fixed EO productionrate), but nonetheless the moderator level needed to be loweredsignificantly in order to maintain optimum operation. Practically,changes in chloride levels less than 0.2 ppm will have an impact oncatalyst performance that cannot be measured precisely. It has beenfound that increasing the inlet water level by 1% typically required areduction in the optimal moderator level between 0.1 and 1.2 ppm, onaverage 0.6 ppm. If a reasonable moderator change is 0.2 ppm, thenchanges in inlet water levels of more than 0.333% would be expected torequire a moderator change of greater than 0.2 ppm and thus wouldrequire re-optimization of the moderator level. These inlet waterincreases of 1% were equivalent to outlet water level increases of 1.1%.Thus, if a reasonable moderator change is 0.2 ppm, then changes inoutlet water levels of more than 0.367% would be expected to require amoderator change of greater than 0.2 ppm and thus would requirere-optimization of the moderator level. Re-optimization may be made tothe moderator level once the inlet or outlet levels change to have anincreased level of more than 0.4%.

The balance inert gases usually present in the feed comprise, forexample, nitrogen, argon, and/or saturated hydrocarbons such as methaneor ethane. If ethane ballast is used, the chloride moderator requiredwill be significantly higher.

“GHSV”, or Gas Hourly Space Velocity, is the unit volume of gas atstandard temperature and pressure (0° C., 1 atm, i.e. 101.3 kPa) passingover one unit volume of packed catalyst per hour. Preferably, if theprocess is carried out as a gas phase process, the GHSV is in the rangeof from 1,500 to 10,000. The reactor inlet pressure is preferably in therange of from 1,000 to 3,500 kPa.

The reaction temperature is typically between about 210° C. and 325° C.from the start of the run to the end of the run. Preferred start and endtemperatures depend upon the particular plant design and the particularcatalyst employed. Typically the reaction temperature is operated at avalue which is sufficient to effect an olefin oxide production of 10,000to 250,000 kg-mole per m³ catalyst bed, preferably up to the end of thecatalyst's lifetime, that is when the catalyst will be exchanged and/orrejuvenated.

The present process may be started-up by using procedures known in theart, for example from U.S. Pat. No. 4,874,879, U.S. Pat. No. 5,155,242,U.S. Pat. No. 6,717,001 and US Pub. Pat. App. 2009/0281339 which areincorporated herein by reference.

The material of the support of the supported silver-containing catalystsmay be selected from a wide range of conventional materials which areconsidered to be inert in the presence of the olefin oxidation feed,products and reaction conditions. Such conventional materials can benatural or artificial and they may include aluminum oxides, magnesia,zirconia, silica, silicon carbide, clays, pumice, zeolites and charcoal.Alpha alumina is the most preferred material for use as the mainingredient of the porous support.

The support is typically porous and has preferably a surface area, asmeasured by the B.E.T. method, of less than 20 m²/g and more inparticular from 0.05 to 20 m²/g. Preferably the B.E.T. surface area ofthe support is in the range of 0.1 to 10, more preferably from 0.1 to3.0 m²/g. The B.E.T. method of measuring the surface area has beendescribed in detail by Brunauer, Emmet and Teller in J. Am. Chem. Soc.60 (1938) 309 316.

The catalyst comprises silver as a catalytically active metal.Appreciable catalytic activity is obtained by employing a silver contentof the catalyst of at least 10 g/kg, relative to the weight of thecatalyst. Preferably, the catalyst comprises silver in a quantity offrom 50 to 500 g/kg, more preferably from 100 to 400 g/kg, relative tothe weight of the catalyst.

The catalyst preferably comprises, in addition to silver, a furtherelement or compound thereof. Eligible further elements may be selectedfrom the group of nitrogen, sulfur, phosphorus, boron, fluorine, GroupIA metals, Group IIA metals, rhenium, molybdenum, tungsten, chromium,titanium, hafnium, zirconium, vanadium, thallium, thorium, tantalum,niobium, gallium and germanium and mixtures thereof. Preferably theGroup IA metals are selected from lithium, potassium, rubidium andcesium. Most preferably the Group IA metal is lithium, potassium and/orcesium. Preferably the Group IIA metals are selected from calcium andbarium. Typically, the further element is present in the catalyst in aquantity of from 0.01 to 500 mmole/kg, more typically from 0.05 to 100mmole/kg, calculated as the element on the total catalyst. Wherepossible, the further element may suitably be provided as an oxyanion,for example, as a perrhenate, sulfate, nitrate, nitrite, borate ormolybdate, in salt or acid form. Salts of Group IA metals or Group IIAmetals are suitable.

Preferred supported highly selective silver-containing catalysts to beused in the present invention are rhenium-containing catalysts. Suchcatalysts are known from U.S. Pat. No. 4,766,105 and U.S. Pat. No.4,761,394, which are incorporated herein by reference. Broadly, thesecatalysts contain a catalytically effective amount of silver, apromoting amount of rhenium or compound thereof, a promoting amount ofat least one further metal or compound thereof and optionally aco-promoting amount of a rhenium co-promoter selected from tungsten,molybdenum, chromium, sulfur, phosphorus, boron, and compounds thereof.More specifically at least one further metal of these rhenium-containingcatalysts is/are selected from the group of Group IA metals, Group IIAmetals, titanium, hafnium, zirconium, vanadium, thallium, thorium,tantalum, niobium, gallium and germanium and mixtures thereof.Preferably at least one further metal is/are selected from the Group IAmetals such as lithium, potassium, rubidium and cesium and/or from theGroup IIA metals such as calcium and barium. Most preferably it islithium, potassium and/or cesium.

Preferred amounts of the components of these rhenium-containingcatalysts are, when calculated as the element on the total catalyst:silver from 10 to 500 g/kg, more preferably from 10 to 400 g/kg, rheniumfrom 0.01 to 50 mmol/kg, further metal or metals from 10 to 3000 mg/kg,and optional rhenium co-promoter from 0.1 to 10 mmol/kg.

More preferably, the rhenium content of these catalysts is at least 0.5mmole/kg, in particular at least 1.0 mmole/kg, more in particular atleast 1.5 mmole/kg, when calculated as the element on the totalcatalyst. More preferably, the rhenium content of these catalysts is atmost 40 mmole/kg, when calculated as the element on the total catalyst.Alternatively, the rhenium content of these catalysts, expressedrelative to the surface area of the support, is preferably at least0.0005 mmole/m², in particular at least 0.001 mmole/m², more inparticular at least 0.0015 mmole/m². Preferably the rhenium content ofthese catalysts is at most 0.1 mmole/m², more preferably at most 0.05mmole/m², relative to the surface area of the support.

As used herein, the quantity of Group IA metal present in the catalystsis deemed to be the quantity in so far as it can be extracted from thecatalysts with de-ionized water at 100° C. The extraction methodinvolves extracting a 10-gram sample of the catalyst three times byheating it in 20 ml portions of de-ionized water for 5 minutes at 100°C. and determining in the combined extracts the relevant metals by usinga known method, for example atomic absorption spectroscopy.

As used herein, the quantity of Group IIA metal present in the catalystsis deemed to the quantity in so far as it can be extracted from thecatalysts with 10% w nitric acid in de-ionized water at 100° C. Theextraction method involves extracting a 10-gram sample of the catalystby boiling it with a 100 ml portion of 10% w nitric acid for 30 minutes(1 atm., i.e. 101.3 kPa) and determining in the combined extracts therelevant metals by using a known method, for example atomic absorptionspectroscopy. Reference is made to U.S. Pat. No. 5,801,259, which isincorporated herein by reference.

The olefin oxide produced may be recovered or removed by using methodsknown in the art, for example by absorbing the olefin oxide in water andoptionally recovering the olefin oxide from the aqueous solution bydistillation. At least a portion of the aqueous solution containing theolefin oxide may be applied in a subsequent process for converting theolefin oxide into a 1,2-diol, a 1,2-diol ether or an alkanolamine.

A description of the olefin epoxidation process, including the variousvessels and process steps is disclosed and discussed with regard to FIG.3 in US Pub. Pat. App. 2009/0234144, which disclosure is incorporatedherein by reference. As disclosed therein, a feed stream comprisingethylene and oxygen is charged to the tube side of shell-and-tube heatexchanger wherein it is contacted with the catalyst bed containedtherein. The shell-and-tube heat exchanger is typically operated in amanner which allows an upward or downward flow of gas through thecatalyst bed. The heat of reaction is removed and control of thereaction temperature, that is the temperature within the catalyst bed,is achieved by use of a heat transfer fluid, for example oil, keroseneor water, which is charged to the shell side of shell-and-tube heatexchanger and the heat transfer fluid is removed from the shell ofshell-and-tube heat exchanger. The reaction product comprising ethyleneoxide, unreacted ethylene, unreacted oxygen and other reaction productssuch as carbon dioxide and water, is withdrawn from the reactor systemtubes of the shell-and-tube heat exchanger and passes to a separationsystem. The separation system provides for the separation of ethyleneoxide from ethylene and carbon dioxide and water—typically this involvesan EO recovery or removal section, a CO₂ absorber section and a CO₂removal section.

The olefin oxide produced in the epoxidation process may be convertedinto a 1,2-diol, into a 1,2-diol ether, into a 1,2-carbonate or into analkanolamine. The conversion into the 1,2-diol or the 1,2-diol ether maycomprise, for example, reacting the olefin oxide with water, suitablyusing an acidic or a basic catalyst. For example, for makingpredominantly the 1,2-diol and less 1,2-diol ether, the olefin oxide maybe reacted with a ten-fold molar excess of water, in a liquid phasereaction in presence of an acid catalyst, e.g. 0.5 to 1.0% w sulfuricacid, based on the total reaction mixture, at 50-70° C. at 1 barabsolute, or in a gas phase reaction at 130-240° C. and 20-40 barabsolute, preferably in the absence of a catalyst. If the proportion ofwater is lowered the proportion of 1,2-diol ethers in the reactionmixture is increased. The 1,2-diol ethers thus produced may be adi-ether, tri-ether, tetra-ether or a subsequent ether. Alternative1,2-diol ethers may be prepared by converting the olefin oxide with analcohol, in particular a primary alcohol, such as methanol or ethanol,by replacing at least a portion of the water by the alcohol.

The olefin oxide may be converted into the corresponding 1,2-carbonateby reacting the olefin oxide with carbon dioxide. If desired, a 1,2-diolmay be prepared by subsequently reacting the 1,2-carbonate with water oran alcohol to form the 1,2-diol. For applicable methods, reference ismade to U.S. Pat. No. 6,080,897, which is incorporated herein byreference.

The conversion into the alkanolamine may comprise reacting the olefinoxide with an amine, such as ammonia, an alkyl amine or a dialkylamine.Anhydrous or aqueous ammonia may be used. Anhydrous ammonia is typicallyused to favour the production of monoalkanolamine. For methodsapplicable in the conversion of the olefin oxide into the alkanolamine,reference may be made to, for example U.S. Pat. No. 4,845,296, which isincorporated herein by reference.

The 1,2-diol and the 1,2-diol ether may be used in a large variety ofindustrial applications, for example in the fields of food, beverages,tobacco, cosmetics, thermoplastic polymers, curable resin systems,detergents, heat transfer systems, etc. The alkanolamine may be used,for example, in the treating (“sweetening”) of natural gas.

Unless specified otherwise, the organic compounds mentioned herein, forexample the olefins, 1,2-diols, 1,2-diol ethers, 1,2-carbonates,alkanolamines and organic halides, have typically at most 40 carbonatoms, more typically at most 20 carbon atoms, in particular at most 10carbon atoms, more in particular at most 6 carbon atoms. As definedherein, ranges for numbers of carbon atoms (i.e. carbon number) includethe numbers specified for the limits of the ranges.

Having generally described the invention, a further understanding may beobtained by reference to the following examples, which are provided forpurposes of illustration only and are not intended to be limiting unlessotherwise specified.

EXAMPLES Example 1 Determination of the Ratio of PPH₂O/VPH₂O andComparison to Relative Surface Concentration of Cesium and Post-MortemCatalyst Performance

The parameters needed to perform the calculations for this example aregiven in Table 1. Note, Steps A-E are plant operating steps andaccompanying operating data. Step F is a post-mortem analysis of thecatalyst.

TABLE 1 Parameters Used for Calculation of the Ratio of H2O PartialPressure to the H2O Vapor Pressure Reactor Inlet Pressure bara 15.0Reactor Outlet Pressure bara 13.3 Total Tube Length m 12.8 Reactor InletH2O Mole frac. 0.0040 Fraction Inlet CO2 Mole Fraction frac. 0.0310Outlet CO2 Mole frac. 0.0437 Fraction

Axial Temperature Profile

The EO Plant (referred to as Plant W) used in this example has multiplethermocouples in multiple tubes of the reactor allowing a direct anddetailed measurement of the catalyst temperature profile. Temperaturesmeasured at 2.1, 6.6, 10.4, 12.3 and 12.8 meters from the inlet of thereactor tube were used for comparison as catalyst samples were collectedfrom each of these positions. The catalyst employed in this example wasa high silver on alumina catalyst having a silver content of about 27%by weight, and containing promoters including cesium, lithium, tungstenand rhenium.

Axial Pressure Profile

Inlet and outlet gas pressures were measured for the reactor and theprocedure outlined in Step B above was used to calculate the pressure ateach of the five axial positions (2.1, 6.6, 10.4, 12.3 and 12.8 metersfrom the inlet of the catalyst bed).

Axial H₂O Partial Pressure Profile

The inlet water mole fraction was available from direct measurementsmade by the Karl Fischer titration method and were confirmed by processcalculations of the inlet water mole fraction. See, e.g., ASTM E203-08.

Direct measurements of the outlet water mole fraction were notavailable, so the reaction stoichiometry and measured CO₂ concentrationswere used to determine the mole fraction of water in the reactor outletas explained in Step C above.

The axial H₂O partial pressure profile was calculated as described inStep C, using the axial temperature and pressure profiles and inlet andoutlet H₂O mole fractions.

Axial H₂O Vapor Pressure Profile

The axial H₂O vapor pressure profile was calculated as described in StepD above using the measured axial temperature profile and the referencedvapor pressure correlation.

Calculation of the Ratio of H₂O Partial Pressure to H₂O Vapor Pressure

The result of the Axial H₂O Partial Pressure Profile was divided by theresults of Axial H₂O Vapor Pressure Profile to give the axial profile ofthe ratio of H₂O partial pressure to the H₂O vapor pressure in thecatalyst bed.

Analysis of Catalyst Samples

X-ray photoelectron spectroscopy was conducted on the samples from 2.1,6.6, 10.4, 12.3 and 12.8 meters from the inlet of the reactor tube andthe relative surface concentration of cesium was determined as describedbelow. Microreactor performance testing was conducted on each sample andthe selectivity difference as described below was determined for eachsample.

Characterization of Spent Catalyst Samples

Calculation of Relative Surface Concentration of Cesium

The following steps provide a full description of the methodologyrequired to determine the surface concentration of cesium (or any otherpromoter) remaining on the catalyst after operation in a reactor.

-   -   i. Once a reactor is shutdown, the catalyst must be removed in        sections and samples clearly labelled so that the precise axial        position (or distance from the inlet) that the sample occupied        in the catalyst bed is known.    -   ii. Samples from multiple sections are then crushed and        homogenized prior to taking small sub-samples.    -   iii. The sub-samples are then analyzed by X-ray Photoelectron        Spectroscopy (XPS) to determine the average concentration of        cesium on the surface of the crushed catalyst.    -   iv. X-ray photoelectron spectroscopy analyses were preformed on        a VG ESCALAB mkII X-ray photoelectron spectrometer.        Non-monochromatized Alka (1484.6 eV) X-rays were used as the        excitation source. The electron kinetic energy analyzer was a        150 degree spherical sector analyzer equipped with a three        channeltron detection system. All spectra were obtained in the        constant analyzer pass energy mode and the pass energy was set        at 50 eV. Prior to analysis, samples were lightly crushed with a        mortar and pestle and mounted onto a sample stub using        double-sided tape. The analysis area was roughly 3 mm×5 mm. The        Al2s peak was used for charge correction and was corrected to        118.9 eV. Linear baselines were used for measuring the peak        heights. Peak intensities were converted to relative molar        values using empirically derived sensitivity factors (SF) and        the following relationship:

${{Relative}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {atoms}} = \frac{\left( {{Cs}\; 3\; d\; 5\mspace{14mu} {{Intensity}/\left( {{Cs}\; 3\; d\; 5\mspace{14mu} {SF}} \right)}\mspace{14mu} {times}\mspace{14mu} 100} \right.}{\left( {A\; 12\; s\mspace{14mu} {{intensity}/\left( {A\; 12\; s\mspace{14mu} {SF}} \right)}} \right)}$

-   -   The relative number of Cs atoms measured for each sample were        normalized to the inlet value.

Performance Testing of Spent Catalyst Samples

-   -   1. Samples taken from each axial position in the reactor were        crushed and 1-5 g were loaded into microreactors.    -   2. Each sample was tested in a microreactor under average        conditions similar to that which the catalyst experienced during        operation in the commercial reactor.    -   3. The selectivity for conversion of ethylene to ethylene oxide        was determined at these operating conditions.

Results for Example 1 are shown in Table 2 and FIG. 1.

TABLE 2 Measured and calculated axial profile values for Plant W.Distance from Inlet m 2.1 6.6 10.4 12.3 12.8 of Reactor Tubes GasTemperature in ° C. 252 259 264 266 244 Catalyst Bed Gas Pressure inbara 14.7 14.1 13.6 13.3 13.3 Catalyst Bed H2O Partial Pressure bara0.089 0.148 0.193 0.215 0.220 H2O Vapor Pressure bara 41.19 45.90 49.6751.87 35.73 Ratio 0.0022 0.0032 0.0039 0.0041 0.0061 Relative Surface1.00 1.00 0.85 0.63 0.50 Concentration of Cesium Selectivity Differencein 0.0 0.0 0.0 −1.5 −9.1 Post Mortem Testing

As shown in Table 2, the temperature at 12.8 meters from the inlet ofthe reactor tube is only 244° C., which is the result of the injectionof cold kerosene near the outlet of the reactor. The results shown inTable 2 dramatically reveal that when the ratio of PPH₂O/VPH₂O is lessthan 0.0040, there is uniform ageing of the catalyst and no selectivitydeficit due to operation under excessive concentrations of water vapor.Where the ratio of PPH₂O/VPH₂O is greater than 0.0040 there is anegative selectivity difference indicating a loss in selectivity.

Example 2 Changing Moderator Levels as Water Levels Change

Example 2 shows the impact on optimum moderator levels with variouscatalysts as the inlet water level changes:

A commercially available, Re-containing catalyst with 13.2 wt % silverwas operated in a laboratory reactor under the following conditions:inlet (dry) composition of 7.3% O₂, 30.9% C₂H₄, and 1.6% CO₂, with 18.3barg inlet pressure, and 3900 gas hourly space velocity, controlling thetemperature to maintain a AEO of 2.50%. When the inlet water level wasincreased from 0% to 0.93% and re-optimized, the optimal moderator leveldecreased 22%. When the inlet water level was increased from 0% to 2.01%and re-optimized, the optimal moderator level decreased 26%. Thecorresponding outlet water levels for the three cases were 0.64%, 1.73%,and 3.07%.

A commercially available, Re-containing catalyst with 17.5 wt % silverwas operated in a laboratory reactor under the following conditions:inlet (dry) composition of 7.3% O₂, 30.9% C₂H₄, and 1.6% CO₂, with 17.8barg inlet pressure, and 4000 gas hourly space velocity, controlling thetemperature to maintain a AEO of 2.49%. When the inlet water level wasincreased from 0% to 0.95% and re-optimized, the optimal moderator leveldecreased 20%. When the inlet water level was increased from 0% to 2.04%and re-optimized, the optimal moderator level decreased 28%. Thecorresponding outlet water levels for the three cases were 0.58%, 1.62%,and 3.10%.

A commercially available, Re-containing catalyst with 27.5 wt % silverwas operated in a laboratory reactor under the following conditions:inlet (dry) composition of 7.3% O₂, 30.9% C₂H₄, and 1.6% CO₂, with 17.8barg inlet pressure, and 3460 gas hourly space velocity, controlling thetemperature to maintain a AEO of 2.48%. When the inlet water level wasincreased from 0% to 0.96% and re-optimized, the optimal moderator leveldecreased 21%. When the inlet water level was increased from 0% to 2.05%and re-optimized, the optimal moderator level decreased 27%. Thecorresponding outlet water levels for the three cases were 0.62%, 1.77%,and 2.88%.

A commercially available, Re- and SO₄-containing catalyst with 17.5 wt %silver was operated in a laboratory reactor under the followingconditions: inlet (dry) composition of 7.3% O₂, 30.9% C₂H₄, and 1.6%CO₂, with 18.3 barg inlet pressure, and 4000 gas hourly space velocity,controlling the temperature to maintain a AEO of 2.48%. When the inletwater level was increased from 0% to 0.96% and re-optimized, the optimalmoderator level decreased 30%. When the inlet water level was increasedfrom 0% to 2.05% and re-optimized, the optimal moderator level decreased28%. The corresponding outlet water levels for the three cases were0.57%, 1.68%, and 3.16%.

A commercially available, Re-containing catalyst with 29.0 wt % silverwas operated in a laboratory reactor under the following conditions:inlet (dry) composition of 7.5% O₂, 25.4% C₂H₄, and 3.9% CO₂, with 19.0barg inlet pressure, and 4900 gas hourly space velocity, controlling thetemperature to maintain a AEO of 1.91%. When the inlet water level wasincreased from 0% to 0.90% and re-optimized, the optimal moderator leveldecreased 11%. When the inlet water level was increased from 0% to 2.01%and re-optimized, the optimal moderator level decreased 10%. Thecorresponding outlet water levels for the three cases were 0.50%, 1.49%,and 2.76%.

Example 3 Determination of the Ratio of PPH₂O/VPH₂O and Comparison toRelative Surface Concentration of Cesium and Post-Mortem CatalystPerformance

The parameters needed to perform the calculations for this example aregiven in Table 3. Note, Steps A-E are plant operating steps andaccompanying operating data. Step F is a post-mortem analysis of thecatalyst.

TABLE 3 Parameters Used for Calculation of the Ratio of H2O PartialPressure to the H2O Vapor Pressure for Plant X Reactor Inlet Pressurebara 22.5 Reactor Outlet Pressure bara 21.1 Total Tube Length m 12.0Reactor Inlet H2O Mole frac. 0.0034 Fraction Inlet CO2 Mole Fractionfrac. 0.0034 Outlet CO2 Mole frac. 0.0135 Fraction

Axial Temperature Profile

The EO Plant (referred to as Plant X) used in this example useswater-cooled reactors and has an outlet gas temperature measurement thatis representative of the catalyst temperature from the 1.5 m position inthe catalyst bed to the outlet of the catalyst bed. Thus, the catalysttemperature for each position at 1.5, 2.7, 5, 10.2, and 10.8 meters wereassumed to be equal to the outlet gas temperature measured for thereactor. The catalyst employed in this example was a silver on aluminacatalyst having a silver content of about 13% by weight, and containingpromoters including cesium, lithium, tungsten and rhenium.

Axial Pressure Profile

Inlet and outlet gas pressures were measured for the reactor and theprocedure outlined in Step B above was used to calculate the pressure ateach of the five axial positions (1.5, 2.7, 5, 10.2, and 10.8 metersfrom the inlet of the catalyst bed).

Axial H₂O Partial Pressure Profile

The inlet water mole fraction was available from direct measurementsmade by the Karl Fischer titration method and were confirmed by processcalculations of the inlet water mole fraction. See, e.g., ASTM E203-08.

Direct measurements of the outlet water mole fraction were notavailable, so the reaction stoichiometry and measured CO₂ concentrationswere used to determine the mole fraction of water in the reactor outletas explained in Step C above.

The axial H₂O partial pressure profile was calculated as described inStep C, using the axial pressure profile and inlet and outlet H₂O molefractions.

Axial H₂O Vapor Pressure Profile

The axial H₂O vapor pressure profile was calculated as described in StepD above using the measured axial temperature profile and the referencedvapor pressure correlation.

Calculation of the Ratio of H₂O Partial Pressure to H₂O Vapor Pressure

The result of the Axial H₂O Partial Pressure Profile was divided by theresults of Axial H₂O Vapor Pressure Profile to give the axial profile ofthe ratio of H₂O partial pressure to the H₂O vapor pressure in thecatalyst bed.

Analysis of Catalyst Samples

X-ray photoelectron spectroscopy was conducted on the samples from 1.5,2.7, 5, 10.2, and 10.8 meters from the inlet of the reactor tube and therelative surface concentration of cesium was determined as describedbelow. Microreactor performance testing was conducted on each sample andthe selectivity difference as described below was determined for eachsample.

Characterization of Spent Catalyst Samples

A similar analysis as done in Example 1 was performed on the spentcatalyst samples of Example 3. Results for Example 3 are shown in Table4 and FIG. 2.

TABLE 4 Measured and calculated axial profile values for Plant X.Distance from Inlet m 1.5 2.7 5.0 10.2 10.8 of Reactor Tubes GasTemperature in ° C. 242.6 242.6 242.6 242.6 242.6 Catalyst Bed GasPressure in bara 22.3 22.2 21.9 21.3 21.3 Catalyst Bed H2O PartialPressure bara 0.104 0.126 0.166 0.255 0.265 H2O Vapor Pressure bara34.95 34.95 34.95 34.95 34.95 Ratio 0.0030 0.0036 0.0048 0.0073 0.0076Relative Surface 1.00 0.94 0.82 0.82 0.79 Concentration of CesiumSelectivity Difference in 0.0 0.0 −0.9 −0.8 −1.8 Post Mortem Testing

FIG. 2 depicts the relationship between selectivity and relative surfaceconcentration of cesium at various ratios of the partial pressure ofwater divided by the vapor pressure of water for Plant X.

The results shown in Table 4 reveal that when the ratio of PPH2O/VPH2Ois less than 0.0040, there is uniform and reduced ageing of the catalystrelative to positions where the ratio of PPH2O/VPH₂O is >0.0040. Wherethe ratio of PPH₂O/VPH₂O is greater than 0.0040 there is a negativeselectivity difference indicating a loss in selectivity.

Example 4 Determination of the Ratio of PPH₂O/VPH₂O and Comparison toRelative Surface Concentration of Cesium and Post-Mortem CatalystPerformance

The parameters needed to perform the calculations for this example aregiven in Table 5. Note, Steps A-E are plant operating steps andaccompanying operating data. Step F is a post-mortem analysis of thecatalyst.

TABLE 5 Parameters Used for Calculation of the Ratio of H2O PartialPressure to the H2O Vapor Pressure for Plant X Reactor Inlet Pressurebara 16.8 Reactor Outlet Pressure bara 15.2 Total Tube Length m 12.0Reactor Inlet H2O Mole frac. 0.0033 Fraction Inlet CO2 Mole Fractionfrac. 0.0548 Outlet CO2 Mole frac. 0.0719 Fraction

Axial Temperature Profile

The EO Plant (referred to as Plant Y) used in this example useskerosene-cooled reactors and has outlet gas temperature measurements,coolant temperature measurements, and a peak temperature differencebetween catalyst and coolant temperature is available. This plant alsoreturns sub-cooled kerosene to the bottom of the reactor to reduce thegas temperature in the reactor outlet. To approximate the axial profileof the gas temperature in the catalyst bed in this case, it was assumedthat the gas temperature at 1 meter from the inlet of the reactor tubewas equal to the coolant temperature of 267° C., measured in the plant.Then the peak temperature difference (PTD) of 16.5° C. was assumed tooccur at a position of 11.3 meters from the inlet of the reactor tube.The following linear interpolation was used to estimate the gastemperature at various positions between the 1 meter and 11.3 meterpoints in the catalyst bed.

Gas  Temperature(z) = Coolant  Temperature  (^(∘)  C.) + Peak  Temperature  Difference  (^(∘)  C.)  times  (z − 1(meter))/(11.3  (meter) − 1(meter))     e.g., Gas  Temperature(3.4  m) = 267.3 + 16.5  times  (3.4 − 1)/(11.3 − 1) = 271^(∘)  C.

Where, Gas Temperature (z) is the gas temperature at the axial positionz in the catalyst bed and is used for positions from 1 to 11.3 metersfrom the inlet of the reactor tube. The value of z is the distance fromthe inlet of the reactor tube. This equation is not applicable for anyportion of the catalyst bed from 0-1 meter as this is a heatup zone froma much lower inlet gas temperature. The equation is not applicable forthe portion of the catalyst bed from 11.3 m to the exit of the tube at12.8 meters due to the effects of the sub-cooled coolant injection nearthe reactor outlet. In this example samples were collected frompositions 3.4, 4.6, 8.2, and 12.5 meters from the inlet of the reactortube. Thus, the equation was used to estimate the temperature for thefirst three positions. For the position at 12.5 meters it was assumedthat the gas temperature was equal to the outlet gas temperature due tothe cooling effects of the sub-cooled coolant.

The catalyst employed in this example was a silver on alumina catalysthaving a silver content of about 13% by weight, and containing promotersincluding cesium, lithium, tungsten and rhenium.

Axial Pressure Profile

Inlet and outlet gas pressures were measured for the reactor and theprocedure outlined in Step 2 above was used to calculate the pressure ateach of the five axial positions (3.4, 4.6, 8.2, and 12.5 meters fromthe inlet of the catalyst bed).

Axial H₂O Partial Pressure Profile

The inlet water mole fraction was available from direct measurementsmade by the Karl Fischer titration method and were confirmed by processcalculations of the inlet water mole fraction. See, e.g., ASTM E203-08.

Direct measurements of the outlet water mole fraction were notavailable, so the reaction stoichiometry and measured CO₂ concentrationswere used to determine the mole fraction of water in the reactor outletas explained in Step C above.

The axial H₂O partial pressure profile was calculated as described inStep C, using the axial pressure profile and inlet and outlet H₂O molefractions.

Axial H₂O Vapor Pressure Profile

The axial H₂O vapor pressure profile was calculated as described in StepD above using the measured axial temperature profile and the referencedvapor pressure correlation.

Calculation of the Ratio of H₂O Partial Pressure to H₂O Vapor Pressure

The result of the Axial H₂O Partial Pressure Profile was divided by theresults of Axial H₂O Vapor Pressure Profile to give the axial profile ofthe ratio of H₂O partial pressure to the H₂O vapor pressure in thecatalyst bed.

Analysis of Catalyst Samples

X-ray photoelectron spectroscopy was conducted on the samples from 3.4,4.6, 8.2, and 12.5 meters from the inlet of the reactor tube and therelative surface concentration of cesium was determined as describedbelow. Microreactor performance testing was conducted on each sample andthe selectivity difference as described below was determined for eachsample.

Characterization of Spent Catalyst Samples

Calculation of Relative Surface Concentration of Cesium

A similar analysis as done in Example 1 was performed on the spentcatalyst samples of Example 4. Results for Example 4 are shown in Table6 and FIG. 3.

TABLE 6 Measured and calculated axial profile values for Plant Y.Distance from m 3.4 4.6 8.2 12.5 Inlet of Reactor Tubes Gas Temperature° C. 271.1 273.1 278.9 243.3 in Catalyst Bed Gas Pressure in bara 16.516.4 15.9 15.4 Catalyst Bed H2O Partial bara 0.130 0.156 0.230 0.312Pressure H2O Vapor bara 55.93 57.66 63.09 35.39 Pressure Ratio 0.00230.0027 0.0037 0.0088 Relative Surface 0.90 1.00 1.00 0.77 Concentrationof Cesium Selectivity 0.0 −0.6 0.4 −4.2 Difference in Post MortemTesting

The results shown in Table 6 reveal that when the ratio of PPH₂O/VPH₂Ois less than 0.0040, there is uniform and reduced ageing of the catalystrelative to positions where the ratio of PPH₂O/VPH₂O is >0.0040. Wherethe ratio of PPH₂O/VPH₂O is greater than 0.0040 there is a negativeselectivity difference indicating a loss in selectivity.

FIG. 3 depicts the relationship between selectivity and relative surfaceconcentration of cesium at various ratios of the partial pressure ofwater divided by the vapor pressure of water for Plant Y.

We claim:
 1. A process for the production of ethylene oxide, whichprocess comprises reacting a feed comprising ethylene and oxygen in thepresence of a catalyst bed comprising silver-containing catalyst loadedin a reactor tube, wherein the presence of water at any point in thecatalyst bed is controlled such that the ratio of the partial pressureof water (PPH₂O) divided by the vapor pressure of water (VPH₂O) is lessthan 0.006.